1. Field of the Invention
The present invention relates to a piezoelectric resonator adapted to generate a third harmonic wave of a thickness extensional vibration mode, and more particularly, relates to a piezoelectric resonator in which the use of a floating electrode makes it possible to suppress the fundamental harmonic wave.
2. Description of the Related Art
In piezoelectric resonators adapted to vibrate in a thickness extensional vibration mode, a variety of piezoelectric resonators utilizing harmonics thereof have been proposed in order to cope with piezoelectric resonators of higher frequencies.
For example, in Japanese Unexamined Patent Publication No. 9-181556, a piezoelectric transducer adapted to generate a third harmonic wave of a thickness extensional vibration mode is disclosed. FIG. 11 is a top plan view of the piezoelectric transducer described in JP 9-181556. In the piezoelectric transducer 51, a vibration electrode 53 extending to the center from the end portion 52a of a piezoelectric substrate 52 is disposed on a first major surface of the strip-shaped piezoelectric substrate 52. On a second major surface of the piezoelectric substrate 52, a vibration electrode 54 extending to the center from the end portion 52b is disposed. The vibration electrodes 53 and 54 are arranged in the middle area of the piezoelectric substrate so as to oppose each other via the piezoelectric substrate 52. The portion where the vibration electrodes 53 and 54 oppose each other constitutes a vibrating portion and the application of an alternating voltage between the vibration electrodes 53 and 54 causes the vibrating portion to vibrate in a thickness extensional vibration mode.
Because the third harmonic wave of a thickness extensional vibration mode is utilized, it is necessary to suppress the fundamental wave as an unwanted spurious component. So, damping layers 55 and 56 are disposed on the first major surface of the piezoelectric substrate 52. Then, the damping layers 55 and 56 are disposed between the above-mentioned vibrating portion and the end portion 52a and between the vibrating portion and the end portion 52b, respectively.
Also, a pair of damping layers with the vibrating portion arranged between them are disposed on the second major surface of the piezoelectric substrate 52. Each of the damping layers on the second major surface is arranged to face the damping layers 55 and 56 disposed on the first major surface, respectively.
The damping layers 55 and 56, and the damping layers disposed on the second major surface are made up of, for example, thermosetting epoxy resins, phenolic resins, solder, and others. It is claimed in JP 9-181556 that in the piezoelectric transducer 51, the vibration of a third harmonic wave can be distributed in the area between the damping layer 55 and the damping layer 56, and the fundamental wave can be effectively suppressed at the damping layers 55 and 56, and others.
On the other hand, in Japanese Unexamined Patent Publication No. 4-216208, a piezoelectric resonator adapted to generate a third harmonic wave of a thickness extensional vibration mode in which the use of a floating electrode makes it possible to suppress spurious components caused by the fundamental wave is disclosed. The piezoelectric resonator described in JP 4-216208 is shown in FIG. 12. In the piezoelectric resonator 61, a vibration electrode 63 is disposed in the middle of the first major surface of a rectangular piezoelectric substrate 62 and a vibration electrode 64 is located in the middle of the second major surface. The vibration electrodes 63 and 64 oppose each other via the piezoelectric substrate 62 located therebetween.
In addition, the vibration electrode 63 is connected to the lead-out electrode 65 disposed along the edge on the short side of the piezoelectric substrate 62, and the vibration electrode 64 is electrically connected to the lead-out electrode 66 disposed along the edge of the short side on the second major surface of the piezoelectric substrate 62.
On the other hand, a pair of floating electrodes 67a and 67b are disposed along the long-side edges on the first major surface of the piezoelectric substrate 62. Also, on the second major surface of the piezoelectric substrate 62, floating electrodes 67c and 67d are disposed so as to oppose the floating electrodes 67a and 67b via the piezoelectric substrate 62 disposed therebetween.
In the piezoelectric resonator 61, the portion where the vibration electrodes 63 and 64 oppose each other constitutes a vibrating portion, and a third harmonic wave of a thickness extensional vibration mode is trapped in the vibrating portion. Also, the fundamental wave is propagated outwardly from the vibrating portion located in the center, but because of the mechanical load of the floating electrodes 67a through 67d and the piezoelectric short-circuit effect, the vibration energy of the fundamental wave is absorbed by the portions in which the floating electrodes 67a through 67d are disposed, and accordingly, the suppression of unwanted spurious components caused by the fundamental wave is supposedly achieved.
Further, in Japanese Unexamined Patent Publication No. 9-139651, a harmonic quartz oscillator provided to suppress the fundamental wave by using floating electrodes is disclosed. The quartz oscillator of JP 9-139651 is explained with reference to FIG. 13.
The quartz oscillator 71 is composed of a rectangular quartz crystal 72. An excitation electrode 73 is disposed in the middle of the first major surface of the quartz crystal 72. Another excitation electrode is disposed in the middle of the second major surface of the quartz crystal 72. The excitation electrode disposed on the second major surface is arranged to oppose the excitation electrode 73 disposed on the first major surface. The excitation electrode 73 is connected to the lead-out electrode 74 disposed around the end portion 72a on the short side of the quartz crystal 72. The lead-out electrode 74 is disposed around the end portion 72a so as to extend over the end surface and the first and second major surfaces.
Also, the other excitation electrode disposed in the middle of the second major surface of the quartz crystal 72 is electrically connected to the lead-out electrode 75 disposed at the other end portion 72b of the quartz crystal 72. The lead-out electrode 75 is disposed at the end portion 72b so as to extend over the end surface and the first and second major surfaces.
Accordingly, the application of an alternate voltage to the lead-out electrodes 74 and 75 produces harmonic waves of a thickness extension mode. In addition, fundamental wave suppressing electrodes 76 and 77 are disposed along the edge of the long sides of the quartz crystal 72. The fundamental wave suppressing electrodes 76 and 77 are disposed along the edge of the long sides on the first and second major surfaces of the quartz crystal 72.
That is, the mechanical damping of only the fundamental wave leaked out of the vibrating portion by the fundamental wave suppressing electrodes 76 and 77 suppresses the fundamental wave.
However, in the above-mentioned piezoelectric resonator making use of a harmonic wave of a thickness extension mode, spurious components caused by the fundamental wave are suppressed only to a certain level, but the fundamental wave can not be fully suppressed.